42 Frederick Ghithrie on Salt Solutions 



the true cryohydrate is formed, and the temperature rises to 

 — 1 1° and remains constant till all is solid. 



The solubilities at 0° and 20° are those given by Kremers. 

 Those and the determination for —11°, when plotted in the 

 usual way, are found to lie on the same straight line. H. Schiff 

 found at 19 c, 5 a saturated solution to contain 03*5 per cent. 

 five-hydrated, or 45*8 per per cent, anhydrous salt. As a 

 cryogen, the temperature —10° was reached. 



§ 182. Citric Acid. — This body presented many difficulties; 

 but as these difficulties occur again with most oro-anic acids 

 of high molecular weight, a special study was made of it. It 

 is peculiarly liable in aqueous solution to supersaturation of 

 the most persistent kind, especially when the solution is at a 

 low temperature. At temperatures and under conditions 

 which are capable of evolving the cryohydrate, the solution- 

 assumes sometimes an almost colloidal form, and shows no signs 

 of eliminating solids unless other means besides mere lowering 

 of temperature are employed. 



From solutions ranging from 10 to 40 per cent, of anhy- 

 drous acid, ice is liberated; and this continues to 42'26 per 

 cent., from which solution a cryohydrate separates at — 9°"2. 

 The following is a somewhat detailed account of the behaviour 

 of solutions containing a greater percentage of acid than 40. 



Two grains of a solution gave 0*8525 of citric acid, or 42*28 

 per cent. This gives a solid at — 9°*2, which at first floats on 

 the residual liquid. The solid consists of massive white ag- 

 glomerated crystals. The crystals are hexagonal, but pre- 

 sent rhomboidal elements, causing the edge of each crystal 

 to be deeply and regularly serrate. When they melt, the 

 rhomboidal crystals are themselves resolved into long slender 

 prisms. A large quantity of such a solution retained its com- 

 position when nine tenths of it had been removed by solidifi- 

 cation, nevertheless, if such a solution be kept perfectly still 

 for many hours at —9°, a few ice spicula may be formed. 



Other solutions, containing respectively 45, 45*9, 50, 50*7, 

 and 51*5 per cent, of the anhydrous acid, were examined. It 

 is only this latter which yields, on cooling, distinct quantities 

 of the original salt : this it does at — 6°, but only if a particle 

 of the original salt be introduced and by diligent stirring. 

 When undisturbed, this 51*5 solution maybe cooled to — 19°"5 

 without any solidification. So prone is this acid to exhibit 

 supersaturation, that solutions both weaker and stronger than 

 the 42*62 may be enriched on partial solidification. Thus a 

 50-per-cent. solution, though already stronger than the cryo- 

 hydrate, may become still stronger by the separation of ice at 

 — 17°. There is therefore a large region of double supersa- 



